Subtractors are normally designed in such a manner that an output voltage which is proportional to a differential voltage is emitted. In this case, the differential voltage results from the difference between two input voltages which are applied to the input of the circuit. The proportionality factor is dependent on the design of the circuit, for example on the gain factor of an amplifier which is used.
High-precision mathematical operations can be carried out using digital computers. However, since the variables to be processed are frequently in the form of continuous signals, for example in the form of an electrical voltage that is analogous to a measurement variable, complex analog/digital converters would have to be provided at the input of digital computers of this type and digital/analog converters would in turn have to be provided at the output of the digital computers. However, this outlay is only worthwhile when a particularly high degree of precision is required.
It goes without saying that it is also alternatively possible to carry out the desired operation using an analog circuit.
A subtractor circuit designed using analog circuit technology needs to be added to the analog computing circuits. By way of example, the document Tietze, Schenk: Halbleiter-Schaltungstechnik [Semiconductor circuit technology], 11th edition, page 771, indicates, in FIG. 11.3, a subtractor of analog design which uses an operational amplifier. In this case, the operational amplifier is connected in a negative feedback path, a resistor coupling the output of the operational amplifier to the inverting input of the latter. The input voltages, the difference between which is to be formed, are supplied to the two inputs of the operational amplifier via respective resistors. The non-inverting input of the operational amplifier is connected to ground via a further resistor.
A classic subtractor circuit of this type having only one operational amplifier operates as an inverting amplifier and provides negative output voltages. A negative supply voltage is needed to operate the operational amplifier.
However, negative supply voltages are not available in every integrated circuit technology or in every integrated circuit application or could additionally be provided only with an impermissibly high outlay.